It was demonstrated in our laboratory that human platelets aggregate when decompressed by reducing barometric pressure to 380 mm Hg. It was also found that hydrodynamic decompression (Bernouilli effect) can induce aggregation of platelets (Murayama, Thrombosis Research 33: 477-485, 1984). Because mountaineer-research physiologist Reinhold Mesner climbed Mount Everest without an oxygen mask by applying intermittently drops of Japanese herbal plant oil (JHP) to his tongue (Nat. Geographic 160: 552-566, 1981), it occurred to us that JHP may contain inhibitors of aggregation of human platelets induced by decreased barometric pressure. It was found that JHP at 0.1 ppm (v/v) inhibited aggregation of human platelets induced by either reduced barometric pressure or Bernoulli effect. Two major constituents of JHP, menthone and menthol, were tested in crystalline form and found to prevent at milliomolar concentrations aggregation of human platelets induced by reduced barometric pressure or Bernoulli effect. Thymol, an unsaturated aromatic derivative of menthol, was also found to be a potent inhibitor of platelet aggregation induced by decompression. These inhibitors were investigated further using an optical aggregomter and several well known chemical inducers of platelet aggregation: ADP, collagen and platelet activating factor (PAF). It was found that the concentration for inhibiting platelet aggregation 50% was in the millimolar range for menthol and in the micromolar range for thymol. Carvacrol (2-methyl-5-isopropylphenol) an isomer of thymol (2-isopropyl-5-methylphenol) was found to be completely inactive. These isomers may prove to be useful probes in the investigation of the nature of receptor(s) on the membrane of platelet. The major goal of the project is to determine at the molecular level the mechanism(s) involved in decompression activation of human platelets, and then find ways and means to induce inactivation by appropriate inhibitors.